The field of microseismics involves the monitoring of small scale seismic events which are induced during hydrocarbon operations and result from small movements along faults and fractures. These are caused by changes in pore pressure or the total stress resulting in release of shear stress or tensile opening. The variations in pore pressure may be due to reservoir depletion or hydraulic fracturing operations.
Hydraulic fracturing is used to stimulate wells by pumping a mixture of fluid and proppant (usually sand) into the well to cause a fracture. When the hydraulic pressure is released the fluid may drain away, leaving the sand holding the fracture open and providing a porous medium through which the oil and/or gas can migrate. Thus, the fractures penetrate into the reservoir to allow a wider area to be drained by the well. However, there is a need for determining exactly where a fracture has been generated so as to determine which areas of the reservoir are being exploited and therefore to plan for drilling future wells, and to determine the movement of the fractures and how they will respond to further hydraulic fracturing. Furthermore, the hydraulically induced fractures may intersect natural fractures in the earth, thus providing further conduits for hydrocarbons. Microseismics can be used to monitor the position and movement of hydraulic fractures and any natural fractures close to the hydraulic fractures.
Unlike in seismic surveying, in microseismics the origin time and location of the events is not known. Furthermore, the monitoring of the seismic events is limited. Monitoring is typically carried out by a line of sensors positioned in a nearby well. Therefore, monitoring is limited by the availability and position of nearby wells in which to place sensors. Furthermore, in order to use a well for monitoring, that well must be shut down for production purposes, which results in substantial revenue loss. There are therefore financial constraints on how many wells can be used for monitoring.
The sensors in the wells detect the arrival of P waves and S waves. The timing difference between the P arrivals (first) and the S arrivals (second), together with a knowledge of the velocity profile of the rocks allow calculation of the distance of the seismic event from the sensor. The polarization can be used to deduce a bearing. Ideally, enough measurements would be taken to construct the moment tensor which is a 3×3 matrix tensor which describes how a fault is deforming and which can be represented by a “beach ball diagram.” However, there are rarely enough measurements available to fully construct the moment tensor.
The microseismic data is usually sufficient to provide the geometry of the fractures as no directionality is required to locate the position of seismic events. Thus the position and extent of the fractures can usually be determined. However, there is usually not enough information available to construct the full moment tensor (beach ball diagram) and hence determine the deformation on the faults.